Leading Edge Manufacturing
Nowadays, most leading edge sections are made of composite materials. Leading edges of these materials provide minimized weight at the same time that maintaining their stiffness.
A well-known method for manufacturing composite leading edge sections uses thermoset prepregs. These come in the form of unidirectional sheets, woven fabrics, or braided material composed of carbon fibers impregnated with uncured thermoset resin. In a first step, a flat lay-up of composite prepreg plies is prepared. Prepregs are formable and very tacky, so that plies stick to each other. Then, the required shape is given to the plies by means of a traditional forming process. After getting the required shape, the section is cured using a male or female tooling. After the curing cycle, the section contours are trimmed getting the final geometry. Finally, the element is inspected by an ultrasonic system to assure its quality.
The cost of a leading edge section manufactured with this prepreg technology is high because the mentioned steps are carried out independently for each section element, and because a final assembly stage is needed.
Another well-known method for manufacturing composite leading edge sections uses Resin Transfer Moulding (RTM) technology with dry fibers. With RTM technology, all dry laminates are formed to the final shape by means of traditional forming processes. Then, all formed laminates are co-injected together in a closed mould.
The RTM technology increases the level of integration of the leading edge section and thus reduces the overall manufacturing costs. Further, with this technology requires only one curing, trimming and inspection process per element.
However, the tooling required to build the whole section is complex, thus making the demoulding process difficult, and the overall manufacturing costs still high.
Leading Edge Influence on Performance
Additionally, aircraft manufacturers are continuously seeking ways of increasing aircraft performance and reducing fuel consumption. One of the main factors when it comes to improving aircraft performance, is the aerodynamic drag on aircraft surfaces.
A significant amount of aircraft drag is caused by turbulent air flow on the aircraft exposed surfaces during flight. Near the aircraft skin the air flow is turbulent mainly due to the following reasons: laminar flow is unstable with respect to small perturbations; and surface imperfections may cause early transition from laminar to turbulence.
Since air laminar boundary layers create less friction at the aircraft surfaces than air turbulent boundary layers, one technique for reducing aircraft drag is to form and maintain a laminar boundary layer over the aircraft external surfaces.
Laminar Flow reduces friction drag and implementation on VTPs and HTPs, would potentially lead to up to 2% aircraft drag reduction.
Current existing methods to form and maintain a laminar flow are:
Natural Laminar Flow (NLF) is obtained by a wing profile that produces a progressive pressure drop (i.e., favourable gradient) resulting in flow acceleration and a delay in transition to turbulence approximately at the point of minimum pressure.
Laminar Flow Control (LFC) which relies on a relatively small amount of air being sucked through a perforated skin to suppress boundary layer instabilities.
Hybrid Laminar Flow Control (HLFC) is a combination of full LFC and NLF as shown in FIG. 1, which relies on: (1) suction being applied to the leading edge section 1, 10-20% of the chord (i.e., ahead of the front spar), to stabilize the flow; and (2) a correctly profiled wing or lifting surface contour, to generate a suitable pressure gradient, thus maintaining the laminar flow aft of the suction area.
Transition from laminar to turbulent flow is delayed by this technique, and may even occur after the 50% chord location, due to the combined effects of the local pressure gradient and Reynolds number.
FIG. 2 shows Hybrid Laminar Flow Control system, to generate a boundary layer by bleeding air through a micro-perforated skin outer surface 11 at the leading edge section 1. The air is ducted beneath the micro-perforated outer surface 11 through a network of chambers 16 to pass through suction holes 15 performed in the inner surface 12 of the leading edge section 1, to be finally exhausted by pipes located at the leading edge section 1.
As suction is limited to the forward part of the wing or lifting surface, HLFC avoids many of the structural problems associated with LFC. It also requires a smaller and lighter suction system. These advantages make HLFC more suitable than full LFC for subsonic transport aircraft. The HLFC technology has also good aerodynamic performance in the fully turbulent mode, which is a significant advantage.
This air suction system of the leading edge section 1 requires a differential pressure distribution over the leading edge surface. This differential pressure distribution is achieved by providing chambers of different size, to obtain different pressure within each chamber, as shown more clearly in FIG. 2C.
As shown in FIG. 2B, the leading edge section 1 with hybrid laminar flow control, is formed comprising a micro-perforated outer surface 11, a perforated inner surface 12, and a plurality of stringers 13 fixed to outer and inner surfaces 11, 12 at specific locations to form chambers 16 of different sizes, to create the chambers 16 allowing air circulation through them.
One of the main problems involved in the implementation of the HLFC technique, is that the components of the leading edge section 1 have to be manufactured separately and then assembled together, such as the manufacturing and assembly processes of these multi-chambered structures with hybrid laminar flow control, are complicated and expensive.
Therefore, it has been detected in the aeronautical industry the need of a method for manufacturing a leading edge section with hybrid laminar flow control, which is faster and simpler that traditional methods, and which is able to reduce the cost and time conventionally required for obtaining said HLFC leading edge sections, at the same time that maintains a minimized weight and a required stiffness for the sections.